1. Field of the Invention
This invention is concerned with a downhole seismic sound source which requires not only electrical control signals but also at least two separate fluids to control its operation, such as may be found in Class 181.
2. Discussion of the Prior Art
This invention finds particular use with downhole geophysical tools such as, for example, the seismic sound source (a.k.a. "the tool") as taught by U.S. Pat. No. 4,671,379, issued June 9, 1987 in the names of William S. Kennedy et al. In that patent, the tool provides a means for exciting the borehole fluid with acoustic waves at some selected depth or depths within the borehole. Inflatable sleeves at each end of the tool provide quarter or half wave reflectors so as to allow the tool to produce standing waves in the borehole at selected resonant frequencies. The desired resonant frequency is controlled by the separation of the inflatable sleeves. The standing waves thus created in the borehole fluid are radiated through the borehole wall and propagate through the earth to seismic detectors that may be located on the surface of the earth and/or in another adjacent borehole.
To operate the sound source, electrical control signals are required to drive a mechanical oscillator that creates a pulsating water head acting against a diaphragm that is exposed to the borehole fluid. The resultant pulsations of the diaphragm set up the desired standing waves in the borehole fluid. The sleeves are inflated by a gaseous fluid, preferably helium, although nitrogen may also be used. The entire assembly is, of course, suspended by a wireline of suitable sizes to support the half-ton-or-so weight of the tool.
In the past, tubing and hoses for the various fluids, as well as the electrical control lines were coiled externally around the wireline. Fluid pressures that are required to equalize the pressure inside the tool with respect to the static pressure existing at the bottom of a borehole that is ten or fifteen thousand feet deep, are on the order of 5000 to 7500 psi. Such pressures required extremely heavy hoses. Wrapped around the wireline, those hoses presented a very awkward, clumsy package that was difficult to handle, to reel in and out of the borehole and to pass over the well-head sheaves.
It is a purpose of this invention to provide a cable head for interfacing a coiled tubing arrangement, that includes a plurality of nested fluid conduits, electrical conductors and a wireline, all coaxially mounted within an outer tube, with a downhole seismic sound source.